The present invention relates to a directional accelerometer and its microlithographic fabrication process. As its name indicates, the directional accelerometer makes it possible to measure a single component of the acceleration of a moving body.
Generally, an accelerometer essentially comprises a moving mass m (pendulum) and means making it possible to measure the force F=m.gamma. due to the acceleration .gamma. of a moving body.
The presently commercially available accelerometers comprise detachable mechanical parts. The volume of such accelerometers is very large, in view of the large number of parts which form the same, as well as their complex manufacturing technology, particularly taking account of the problem of positioning the various elements constituting said accelerometers and the assembly thereof.
The use of techniques resulting from semiconductor technology is at present being developed with the aim of reducing the size of such accelerometers, together with their production costs, particularly by batch fabrication on the same flat substrate. Such an accelerometer fabrication procedure is described in an article by K. PETERSEN by Proceedings of the IEEE, Vol. 70, No. 5, May 1982.
In FIG. 1 is shown in longitudinal sectional form, the basic diagram of an accelerometer constructed in accordance with this novel procedure. The accelerometer comprises a substrate 2, made e.g. from silicon or glass, which has a recess 4. Onto the upper surface of the substrate is deposited, e.g. by vacuum deposition, a flexible thin layer 6 in the form of a beam and more particularly made from silica, doped silicon or metal, which overhangs the recess 4 formed in the substrate. This beam, which is able to deform or move in a direction perpendicular to the surface of the substrate represented by direction z, supports at its free end a seismic mass 8.
The measurement of the displacement of mass 8, which is proportional to the component of the acceleration in direction z which it is wished to measure, either takes place through the measurement of the variation of the capacitance of the capacitor defined by the thin layer 6 in the form of a beam and substrate 2, or with the aid of a piezoresistive element attached to said thin layer.
The above accelerometer in fact corresponds to what can best be done at present on a silicon plate, but suffers from a number of disadvantages. In particular, the flexible beam 6 can give rise to internal stresses leading to the bending of the beam, even when there is no acceleration. These stresses, which are very difficult to control, are particularly due to the accelerometer fabrication process. In addition, these stresses vary with the temperature.
Another disadvantage is caused by the stack of different materials, which have different expansion coefficients and which also lead to the formation of inadmissible stresses.
Moreover, as the mass 8 attached to the flexible beam 6 is off center with respect to the beam axis, this type of accelerometer is also sensitive to the component of the acceleration in a direction parallel to the surface of substrate 2, such as direction y. However, a good directional accelerometer must only be sensitive to a single component of the acceleration to be measured. This phenomenon is further aggravated when the beam 6 bends in the absence of acceleration, due to stresses within the actual beam.
Moreover, due to the lack of symmetry of the structure of the accelerometer, it is very difficult to perform a differential measurement of the displacement of mass 8. However, it is not possible to carry out a precise measurement of the position of the beam without such a procedure.
In order to obviate this lack of symmetry, it would be possible to join a second symmetrical substrate to the first with respect to the thin layer 6. Such a device is described in the article in IEEE Transactions on Electron Devices, Vol. ED-26, No. 12, December 1979, New York 45A, pp. 1911-1917 entitled "A batch fabrication silicon accelerometer" by L. N. ROYLANCE et al. In the case of said device, the problem of internal stresses occurs at the actual substrate, as a result of the sealing. In addition, this process is complicated and costly.
Another disadvantage of these accelerometers is their limited sensitivity. Thus, the dimensions of the thin layer 6 in the form of a beam are fixed, so that it is difficult to increase the seismic mass 8, the latter having at the most a thickness of a few microns.
A possible variant of the accelerometer shown in FIG. 1 consists of having two ends of a thin layer 6 fixed to the substrate, which makes it easier to fix the starting position of mass 8, even when there are stresses within the said layer. However, such an accelerometer has a much more rigid structure and consequently a reduced sensitivity.